ABSTRACT
The effluent quality index (EQI) can be defined as a single numeric value, which reflects the overall wastewater quality related to its constituent parameters. The major objective of the present study was to obtain an index from combination of different parameters that represents the overall quality of effluent in the form of single number. In this study, by analyzing of three year measured data of waste water in ASSALUYEH and MAHSHAHR petrochemical companies, it was found that only a limited number of parameters, required to be measured, have variable numbers, the other parameters are almost constant and unchanged. Therefore, the relevant index should be a set of these variable data that depend on the nature and operating conditions of production process. In this regard, the EQI index has been obtained based on a simple model using multiple linear regressions of these variable parameters with different specific weight coefficients.

Introduction
Aquatic systems provide a wide range of valuable ecosystem services, including drinking water, food provisioning, recreation, water purification, and climate regulation. As a result, their surroundings are historically associated with human occupation, and environmental degradation is often observed. One of the major disturbances affecting water bodies in urbanized watersheds is related to poorly treated or untreated wastewater spills, especially in developing countries. This can be caused by irregular connections of sewage to the storm water network, leaks and overflows from the sewerage system itself, bad performance of the wastewater treatment plant and its outfall pipe, or inadequate installation of septic tanks. The excess of organic matter and nutrients in wastewater entering the aquatic systems can cause an increase in the trophic state, algal blooms, and oxygen depletion. This results in an overall deterioration of water quality and ecological imbalance. Discharge of wastewater can therefore negatively affect the chain of services and goods provided by water bodies and result in irreversible damage to the ecosystem.
Based on this, laws and requirements have been established all over the world according to the climate of the regions. Also, in Iran, these requirements have long been defined and implemented as permissible limits and standards set by regulating body DOE (department of environmental) of Iran. Due to the large number of these parameters and the need to measure them in short time intervals, every industry after a period of measurement, identifies the nature of the composition of its production effluents and limits its routine measurement to the identified parameters. Of course, this limitation is only for controlling the performance of the relevant process unit.
With the development of numerous industries, the increasing use of water resources and the discharge of pollutants into the environment, particularly industrial wastewater, have become extremely important. The purpose of this study was the assessment of treated wastewater attributed to the petrochemical industries as reliable and sustainable water resources based on the effluent quality index (EQI).

A lot of research has been done to achieve an equivalent index for the quality of water, sanitary and wastewater in the world, among which it can be mentioned to find the index of the treated effluent from the treatment plants in order to achieve the efficiency of the treatment plant [1-5]. The use of WWQI (Wastewater Quality Index) is used as an evaluation tool in many countries of the world, including Iran, to check the quality of treated wastewater discharged into rivers and evaluate the quality of water flowing in surface water channels.


Methodology
In each of the production processes of the petrochemical industry, according to the type of product, the characteristic compositions of the produced effluent can be identified after a certain period of time and under different operating conditions including start-up, normal operation and shutdown. In this research, the measured data of the effluents of petrochemicals in the two areas of MAHSHAHR and ASSALUYEH were tested and analyzed and the results show that except COD, PH, Turbidity, Oil, TDS and Phenol parameters, other characteristics have insignificant changes.
In this article, using the available data and inspired by the methods of obtaining an index in the "Water Quality Indicators" handbook [6], it has been tried to aggregate the above data into a quantitative index that indicates the level of pollution. By applying this number, evaluation process by governing and regulatory organizations is facilitated.
For this purpose, taking into account different equation methods, a first-order linear equation with a combination of distinct parameters and different weight coefficients was selected as the best equation, and after many trials and errors, the equivalence index was defined.
EQI index is generally defined with the linear format, which consists of the sum of several sub-indexes; these sub-indexes are obtained by numerically dividing by the amount of the standard limit of discharge into the sea and then the power of 2. The weights assigned to each of these sub-indices are defined according to the region and the importance of each parameter in Table 2.

In order to convert the obtained EQI quantitative index into a qualitative index, a category has been defined according to Table 3, which is currently available to higher institutions as a reference.
As an example of calculations, some real data were used after being included in the EQI index formula as described in Table 4.

Conclusion
According to the study conducted and the actual data measured from the effluent output of petrochemical companies in Mahshahr and Assalouye Special Economic Zone, it was concluded that with the current conditions and no change in the process, the EQI equivalent index calculated in this research as a tool Evaluating and sometimes ranking companies is very useful. Even after converting this numerical value into a qualitative expression classified as "excellent", "good", "healthy", "unhealthy", "very unhealthy" and "dangerous", is also tangible for the general public. In the case of process changes, it is necessary to measure a continuous time period of all the parameters required by the environmental organization to determine whether another pollutant parameter is added to this formula or possibly a parameter should be removed, in which case the formula related to the calculation and the equivalent index must be changed and defined again.